A YEAR ago, China appeared to be pursuing a more aggressive policy toward its Pacific maritime disputes with a half-dozen nations. It engaged in some high-stakes jockeying with Vietnam over disputed maritime claims in the South China Sea, moving an oil drilling rig to the Paracel Islands and declaring an exclusion zone around it. That followed its declaration in 2013 of an air defense exclusion zone over islands in the East China Sea that are also claimed by Japan; the United States and Japan promptly flew planes through the zone to demonstrate they did not recognize it.

But in recent months, Beijing quietly — and significantly — shifted tactics. It toned down its confrontation with Japan, agreeing to a handshake between President Xi Jinping and Prime Minister Shinzo Abe last November, and pulled back the drilling rig that infuriated Hanoi.

Admiral Harry B. Harris Jr., the commander of the U.S. Pacific Fleet, delivered a speech in Canberra, Australia, on March 31 that offered a revealing and unusual description of how China is dredging its atolls, turning coral reefs into more permanent and larger islands. He said China is engaged in “unprecedented land reclamation,” an effort to build artificial lands by “pumping sand on to live coral reefs — some of them submerged — and paving them over with concrete.”

“China has now created over four square kilometers of artificial landmass,” he said. “China is creating a Great Wall of sand, with dredges and bulldozers, over the course of months.” In effect, China seems to be fortifying these specks of land for something in the future.

Chinese Foreign Minister Wang Yi said March 8 that China was just “carrying out necessary construction on its own islands and reefs,” and insisted that it “does not target or affect anyone.” China, he added, seeks “to bring harmony, stability and prosperity to the neighborhood.” Nice words, but pay attention to deeds. What is the purpose of the digging? Are these reinforced outposts going to be used someday for military forces that would attempt to enforce China’s vaguely drawn “nine-dash line” territorial claim that encompasses most of the South China Sea? As Admiral Harris put it, “the scope and pace of building man-made islands raise serious questions about China’s intentions.”

Although the dredging has not raised the same alarms among China’s neighbors as the earlier moves, the pace of work has caught their attention. The Vietnamese foreign minister, Pham Binh Minh, told the Financial Times that the digging has “really risked escalating tension in the area.”

The United States does not have a direct dispute with China over any of the maritime territories, but it does have an enormous stake in the stability of the region and in the defense of Japan. China has wisely turned away from aggressive confrontation in recent months, but that may not be the whole story. Admiral Harris has sounded a valuable warning. Vigilance is required to make sure that China has not simply found another way to exert its will, with a Great Wall of sand.